The invention relates to devices for prestressing concrete works using relatively long stretched sinuous cables formed by bundles of several metal strands (simple or composite wires), which bundles are housed in sheaths themselves buried over at least a part of the length in the concrete works, more especially during casting of same.
It also relates to the methods for implementing these prestress devices.
It relates more particularly, among these devices and methods, to those for which each cable strand is itself contained in a plastic material tube with preferably interpositioning of grease.
It relates more particularly still to the case where the cables considered are intended to support high tension forces.
In such a case, it is particularly advantageous to be able to tension the cables strand by strand.
Such a measure in fact allows actuating cylinders or jacks to be used for such tensionings which are lighter and more compact than those required for tensioning in a single go, which allows generally available devices to be used for operating these actuating cylinders which lend themselves in particular to automated control.
To make such tensioning possible strand by strand, the different tubes must remain spaced apart transversely with respect to each other so as to avoid them rubbing against each other in their mutual contact positions, which would risk tearing them, placing the corresponding metal strands in direct contact and finally preventing the individual tensioning.
Since the cable considered is sinuous and not rectilinear, such transverse spacing of the different strands can only be obtained by providing special spacers in the sheath, at least at its bends.
These spacers, generally formed by ties or by moulded pieces recessed by as many housings as there are strands to be spaced apart, are relatively costly and difficult to position.
In French patent No. 2 511 721 of the applicant, it has been proposed to form a curved and highly prestressed cable section for connecting therebetween two rectilinear portions of a stretched cable, this section being itself formed by a bundle of parallel metal strands arranged in the way described above.
In other words, the bundle is contained in a sheath itself buried in a concrete mass, each strand is housed in a plastic material tube and the different strands are stretched while the tubes are held transversely spaced apart from each other in an arrangement organized by means of spacing members.
Furthermore, a solidifiable cement mortar is injected into the sheath after the tubes with their cable strands are positioned therein.
In this patent, the sheath is formed by a metal tubular section curved in the form of a single relatively short arc of a circle resisting axial compression so as to absorb the prestress tension exerted by the strands between its two ends and it has been proposed, for relieving the metal forming this tubular section during application of said prestress, to solidify the cement mortar before tensioning of the strands.